The present invention relates to thermoplastic compositions having improved impact strength. More particularly, it relates to polycarbonate resin mixtures having improved impact strength at room temperatures.
Aromatic carbonate polymers are well known, commercially available materials having a wide variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl) propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. Generally speaking, aromatic polycarbonate resins offer a high resistance to the attack of mineral acids, may be easily molded, are physiologically harmless as well as strain resistant. It is also well known that polycarbonate resins have high impact strength below a critical thickness of between about 1/8 and 1/4 inch. Above this average thickness the impact strength of polycarbonate resins diminishes and is accompanied by a change in failure mode from ductile to brittle. Moreover, the impact strength of polycarbonate resins decreases rapidly as the temperature decreases below about -5.degree. C. and also after aging the polymers at elevated temperatures above about 100.degree. C. These characteristics consequently limit the fields of application for unmodified aromatic polycarbonate resins.
Accordingly, in an effort to improve the impact resistant behavior of polycarbonate resins, it has been proposed to add modifiers to the polycarbonate. In Goldblum, U.S. Pat. No. 3,431,224, assigned to the same assignee as the present invention, for example, it is proposed to add modifiers, in certain proportions, the modifiers comprising at least one member of the class consisting of polyethylene, polypropylene, polyisobutylene, a copolymer of ethylene and an ethyl acrylate, a copolymer of ethylene and propylene, a cellulose ester, a polyamide, a polyvinyl acetal, an alkyl cellulose ester, and a polyurethane elastomer. Although the results with such modifiers are generally excellent in thin sections, e.g., in parts of 1/8 inch thickness, there is a tendency for failure to occur with these modifiers in thicker molded parts of 1/4 inch or more, and such failure is of the undesirable brittle type.
Nakamura, et al., discloses in U.S. Pat. No. 3,864,428 compositions comprising blends of aromatic polyester, aromatic polycarbonate and butadiene based graft copolymers, wherein the polycarbonate comprises from 10 to 75 weight percent of the overall blend, which are said to possess good room temperature impact strength.
Fromuth, et al., in U.S. Pat. No. 4,180,494, discloses high impact strength compositions comprising about 25 to 95 percent by weight aromatic polyester, about 1 to 8 percent by weight of an aromatic polycarbonate and the balance to make 100% of a core shell copolymer having a butadiene based core.
Farnham, et al., in U.S. Pat. No. 4,086,202 discloses that the impact resistance of poly(alkylene terephthalate) resins is improved by the incorporation of a multiphase composite interpolymer having an elastomeric first stage and a thermoplastic final stage.